<!DOCTYPE html>
<html>
    <head lang="en">
        <meta charset="UTF-8S">
        <title></title>

    </head>
    <body>
        <script>
        function setValue(){
            var lable =document.getElementById("label");
            var textbox = label.control;
            textbox.value = "323423"

        }
        </script>
        <form >
            <label id="label">
                邮编:
                <input id ="text_zip" maxlenth="6">
                <small>请输入六位数</small>
                
            </label>
            <input type="button" value="设置默认值" onclick="setValue()">

        </form>
        
    </body>
</html>